There are a variety of known electromagnetic techniques. Geophysical electromagnetic (“EM”) techniques can be effective in determining the electrical conductivity of soils, rocks and other conductive material at depths up to about one kilometer. Conductivity distribution with depth is of great interest in mapping base metals and uranium deposits, aquifers and other geological formations. Active geophysical EM methods generally involve generation of a magnetic field by applying a periodic current to a transmitter coil system placed near the earth's surface. This primary magnetic field induces electrical currents in the ground, and the secondary magnetic field produced by these currents is measured to provide information about ground conductivity distributions.
The secondary magnetic field signal may be measured using either a receiver coil system (which can measure up to three orthogonal components of the magnetic field time derivative dB/dt), or a magnetometer (which measures the magnetic field B). The received analog signal may then be amplified, filtered, and digitized by a high-resolution high-speed analog-to-digital converter (“ADC”), and the data may be stored along with positioning information obtained from a Global Positioning System (“GPS”). Data post-processing can involve electrical and physical modeling of the ground to generate the geophysical conductivity contour maps.
EM measurements can be done either in the frequency domain or time domain. In frequency-domain electromagnetic (“FDEM”) measurements, the transmitter coil generally continuously transmits an electromagnetic signal at fixed multiple frequencies, while the receiver coil may measure the signal as a function of time. The measured quantities may include either signal amplitude and phase, or equivalently, the in-phase and in-quadrature amplitudes as a function of frequency.
In time-domain electromagnetic (“TDEM”) systems, a pulse of current may be applied to the transmitter coil during an on-period, generating the primary or transmitted EM field, and then switched off during an off period. The secondary signal may be measured at the receiver coil as a function of time. The signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, may be utilized to yield conductivity contour maps. U.S. Pat. No. 7,157,914 shows an example of a TDEM system.
EM methods can encompass both ground-based and airborne applications using airplanes and helicopters. Airborne methods may be convenient for large area surveys and have been used for exploration of conductive ore bodies buried in resistive bedrock, geological mapping, hydrogeology, and environmental monitoring. By way of non-limiting example, in some airborne electromagnetic (“AEM”) systems, the data may be acquired while the airplane or helicopter flies at nearly constant speed (for example, up to 75 m/s or 30 m/s, respectively) along nearly parallel equally spaced lines (for example, 50 m to 200 m) at close to constant height above ground (for example about 120 m or 30 m, respectively). Measurements can be taken at regular intervals, for example in the range 1 m up to 100 m.
In designing a helicopter mounted time-domain electromagnetic (“HTEM”) system, there are a number of desired features including: high signal-to-noise ratio (“SNR”), high conductance discrimination, and high spatial resolution both laterally and in depth. High signal-to-noise ration can be accomplished by lowering system noise, or by increasing the signal at the receiver coil. One method of increasing the signal means may be increasing the primary magnetic field.
For a point far away from the transmitter coil, the magnetic field is proportional to the magnetic dipole moment of the coil and inversely proportional to the cube of the distance from the coil. The magnetic dipole moment of a coil is the product of N*I*A where N is the number of turns, I is the current, and A is the coil area. The inductance of a coil is proportional to N2×D, where N is the number of turns and D is the diameter of the coil. The voltage induced in the receiver coil by a magnetic field B is given by N*A*dB/dt, where the coil sensitivity N×A is the product of the coil number of turns N and the coil area A, and dB/dt is the time-derivative of the magnetic field.
Whenever the survey objective is to map near surface conductivity, a small magnetic dipole moment with fast turn-off may be appropriate, in which case the number of turns in the transmitter coil is generally smaller, thus yielding a reduced magnetic dipole moment and inductance. Conversely, for the detection of conductors at greater depths, it may be desirable to have a longer off-period, and more importantly, to increase the transmitter coil magnetic dipole moment.
Whenever an increase in the magnetic dipole moment may be warranted, it is necessary to increase either the current I, the number of turns N, or the area of the transmitter coil A. The electrical power supply from a single engine helicopter may be limited by the helicopter generator unless an auxiliary power supply is used. Also, a limiting factor for the amount of current in the transmitter coil is the electrical resistance of the coil and tow cable. For a fixed-length of cable, the power, P, from the helicopter electrical supply is dissipated approximately as the square of the current times the resistance (P=I*I*R). Decreasing the resistance will increase the current by the square root of the decrease. Decreasing the resistance in the loop may be accomplished by heavier gauge wire with its corresponding increase in weight as the electrical resistance is approximately proportional to the length times the resistivity divided by the cross sectional area of the wire. The weight of the transmitter coil is also proportional to the length of the cable, and therefore is proportional to the number of turns N or the square root of the transmitter coil area A. Since the weight of the transmitter coils increases as the square of the current I, and linearly with the number of turns N, and as the square root of the area A, for a given towing weight capacity of the helicopter, the one way to increase the magnetic dipole moment of the transmitter coil may be to increase the area A. Another factor to consider when optimizing the transmitter coil I, N, and A is the requirement of a short turn-off time in time-domain measurements, which can require a low inductance of the transmitter coil, the inductance being proportional to the square of N and to the square root of the transmitter coil area.
However, increasing the transmitter coil diameter may reduce aerodynamics and increase drag. Large structures may be stressed during take-off and landing, and therefore there is generally a limit for the size of rigid structures that can be deployed without breaking apart. Reinforcing the structure so that it does not break during take-off and landings may mean an increase in the weight of the structure. Additionally, maintaining the transmitter coil shape during flight can be very important to provide a fixed magnetic dipole moment, in order not to degrade the quality of the measurements. Thus, the requirement for an increased magnetic dipole moment can require careful balancing of many factors.